Telecommunication systems, particularly mobile communication systems, are developing at an increasing pace. While the telecommunication systems have evolved, services provided via the systems also have been under development. Many services, for example, services involving financial transactions, employ digital certificates, hereinafter called certificates, to dynamically establish a level of trust between the parties, i.e. a two-way trust relationship between a service provider and a subscriber using the service. By issuing certificates to subscribers an operator can also offer authorization and accounting to services provided by other service providers. A certificate is a proof normally supplied by a third party, usually a certification authority, to confirm that a digital signature belongs to a certain person or organization and is valid.
One of the problems associated with certificates in a mobile communication system originates from the subscribers' ability to move within the service area of the system. A subscriber in a service area of a visited network, i.e. another network than his home network, may need a certificate issued by the operator of the visited network, for example when he wishes to use services provided by a service provider who has a contractual relationship with the visited network operator. However, the subscriber has a contractual relationship with his home network operator, and therefore, the home network operator should have some control over issuing certificates in visited networks.